This invention relates generally to hydraulic drive systems and more specifically to a hydraulically driven vehicle wherein motors on two of the wheels can be optionally powered as needed. Generally, an off-highway vehicle includes a load carrying body supported on both a pair of main traction wheels and a pair of steerable wheels. When operating under heavy load in soft soil conditions, it is desirable to have the steerable wheels powered as well as the main traction wheels. Once the loaded vehicle reaches a hard surfaced road, however, it is better to be able to deactivate the hydraulic drive from the steerable wheels and rely on the main drive wheels for propulsion. Deactivation on hard surfaced roads allows the top speed of the vehicle to be increased by at least a third since the main pump supplies fluid to only the main traction motor. With our invention the auxiliary drive can be used only as needed.
Various hydraulically driven vehicles are known. The U.S. Patent to Jennings, et al (No. 3,736,732) shows an auxiliary drive system for driving the steerable rear wheels of a combine. U.S. Pat. No. 3,997,017 discloses an auxiliary hydrostatic front wheel drive system having fluid pressure actuated clutches to effect a driving combination between the motors and wheels. U.S. Pat. No. 4,072,009 presents the pressure system for driving the wheels of a skid steered front loader. U.S. Pat. No. 4,140,196 shows an auxiliary drive system for optionally powering the steerable wheels of a four-wheel vehicle.
Another type of auxiliary drive system is disclosed in a copending application Ser. No. 068,756, filed Aug. 23, 1979, assigned to the same assignee as the present invention.
None of the above disclosed systems have the simplicity and inherent advantages of our invention. The powered auxiliary axle is engaged or disengaged by an electrical switch on the operator's console. The switch activates a solenoid which controls hydraulic valves located directly adjacent the respective wheel drive motors. During the disengaged mode, the steerable wheels drive the auxiliary motors, causing them to pump oil around a closed loop circuit. Our invention keeps pressure drops in the closed loop circuit to a minimum, thereby reducing energy losses under two wheel drive conditions.